The present invention concerns a drive system and a method of driving a vehicle.
To use a conventional clutch mechanism which disconnects the input shaft of the gear box from the combustion engine during gear changes in the gear box involves disadvantages. When a stationary vehicle starts, the discs of the clutch mechanism slide against each other, thereby heating the discs. This heating results in increased fuel consumption and wear of the clutch discs. A conventional clutch mechanism is also relatively heavy and expensive. It also occupies a relatively large space in the vehicle. Hydraulic moment converters also result in losses.
Hybrid vehicles may be driven by a primary motor, which may be a combustion engine, and a secondary motor, which may be an electric machine. The electric machine is equipped with at least one energy storage for storing electric energy and control equipment for controlling the flow of electric energy between the energy storage and the electric machine. The electric machine may alternately work as a motor and a generator depending on the operation state of the vehicle. When the vehicle is braked, the electric machine generates electric energy which is stored in the energy storage. The stored electric energy is used later, for example, for driving the vehicle and operating different auxiliary systems in the vehicle.
Swedish patent application SE 1051384-4, which has not been made public, shows a hybrid drive system with a planetary gear including three components, namely, a sun wheel, a planet wheel holder and a ring wheel. One of the three components of the planetary gear is connected to an output shaft of the combustion engine, a second component of the planetary gear is connected to an input shaft of the gear box and a third component of the planetary gear is connected to a rotor of an electric machine. The electric machine is connected to an energy storage such that it alternately works as a motor and a generator. The rotation speed of electric machines may be controlled in a stepless manner. By controlling the rotation speed of the electric machine, the input shaft of the gear box may be rotated at a desired rotation speed. With a hybrid system according to SE 1051384-4 no clutch mechanism needs to be used in the drive line of the vehicle.
In conventional hybrid vehicles with a clutch, the combustion engine cannot be used for charging the energy storage when the speed is so low that the vehicle can not be driven with the clutch in a closed position. When such a hybrid vehicle in the form of, for example, a bus, is driven at a very low speed during, for example, queuing, there is a risk that it will be driven with an engaged gear and the clutch in an open position. If at the same time, the energy storage is loaded by other electrically operated assemblies in the vehicle, such as, for example, a compressor of AC-equipment, the energy storage may be discharged quite quickly. In this case, the vehicle is stopped, the engaged gear must be disengaged, and the clutch must be closed in order to charge the energy storage. Alternatively, the vehicle must begin to slip heavily on the clutch to charge the energy storage.